1. Field of the Invention
This invention relates to electrodes, or catheters, and, more particularly, to unipolar electrodes adapted for use in combination with cardiac pacers for low power drain stimulation of a patient's heart.
2. Description of the Prior Art
In the approximately 14 years that implantable cardiac pacing devices have been available, a great number of important technological develpments have taken place which have greatly increased the efficiency, reliability and lifetime of implantable pacers. The pacing devices themselves, comprising the battery power source and the electronic circuitry, have over this period of time incorporated substantial changes. In addition, the electrodes, or cathaters by which the pacer produced stimulus pulses are delivered to the heart tissue, have also experienced significant design changes. Throughout the history of development of cardiac pacing systems, a foremost objective has been to utilize the available power in the implanted device to the maximum possible extent. In order to do this, it has become recognized that the system, comprised of both the pacer device and the electrode, must be optimally matched in order to most efficiently deliver the stimulus pulses.
Another area of great concern where improvement is still sought is in the area of providing and maintaining good electrode contact at the heart, so as to ensure good stimulus threshold. The electrode design must be such that the stimulus pulses are delivered effectively to the heart tissue. This requirement generally suggests that the distal tip of the electrode must be sufficiently well positioned so that the best possible threshold is obtained. However, positioning can be a difficult procedure, and the physician would prefer that the positioning not be so critical. An alternate approach is to make the electrode contact surface large enough so that the position of the electrode tip is not critical. This possibility is not a feasible one, since a large surface electrode would present a very low output resistance to the pacer, and cause substantial and unnecessary current drain. Further, the large surface would result in the current density of the delivered stimulus pulse being very low, and the lower the current density the less chance of cardiac stimulation. For a unipolar electrode system, the electrical field emanates from the single contact surface and passes through the ventricular wall to the distant indifferent electrode. It is well understood that the larger the surface of the contact element, the more dispersed is the emanating electrical field, and consequently the smaller the ability of a pulse of a given power to produce cardiac stimulation.
To overcome these difficulties, a wide variety of electrode designs have been developed and utilized. The predominant design philosophy currently is to reduce the contact surface, so as to increase current intensity and to achieve reliable stimulation with lower output. However, mere reduction of the electrode surface carries with it the adverse feature that positioning may be more critical. To overcome this, some electrode designs have incorporated features whereby the distal tip actually penetrates into the endocardium, as for example using a type of corkscrew. However, this general arrangement isn't satisfactory in that it results in muscle damage and tissue build-up which is detrimental to long term optimization of stimulus thereshold. Further, even where the distal tip of the electrode is held firmly in contact with the endocardium, it remains a question whether the available contact surface is properly positioned relative to the endocardium. For example, a typical contact tip as found in the prior art has the form of a cylinder with a closed distal end, and for this shape the stimulus current delivery is inefficient depending upon whether the heart tissue at the point of contact is relatively flat, convex or concave, and how the tip interfaces with the heart tissue. Frequently in placement of an electrode with such a tip, only a corner between the end and the side cylindrical wall will be actually positioned in contact with the heart tissue, which can result in a high stimulus threshold. It is thus seen that the solution to the problem requires not simply making the electrode small, but both shaping and positioning it at the distal end of the electrode so that positioning is relatively non-critical.